Studies have been carried out to determine what factors may be important in preventing functional deterioration in a diseased or damaged kidney. An experimental model of glomerulonephritis in rats, nephrotoxic serum nephritis, was utilized. It was found that animals with this disease were protected from functional deterioration and histological damage by feeding them a phosphate restricted diet. In contrast animals maintained on a normal diet developed progressive renal failure and death from uremia. One mechanism by which phosphorus restriction could be of benefit would be the accompanying hypercalcemia which could suppress parathyroid hormone secretion and prevent renal parenchymal calcification. To test this possibility thyroparathyroidectomy was performed on rats with nephrotoxic serum nephritis. This procedure was found to be equally as protective as phosphorus restriction. However, animals with selective parathyroidectomies, thyroids intact, did not experience any protection and died of uremia. In contrast animals with selective thyroidectomy did not develop progressive renal insufficiency unless they received thyroid replacement. It is concluded that thyroid deficiency and phosphate restriction through unknown mechanisms have a profound effect on experimental glomerulonephritis in regards to preventing functional deterioration.